Abstract
The baseline situation for the three regularly occurring species of marine mammals at the Project Area, Harbour porpoise, Harbour and Grey seal, has been described on the basis of habitat models applied to the available telemetry data and acoustic data recorded during summer 2009. Impacts on the regional populations of the two species have been assessed by linking the identified habitats to noise-related disturbance using in situ measurements together with a frequency-related impact assessment.
Existing data on the abundance and distribution of Harbour porpoises and Harbour seals in the Project Area included Satellite telemetry data on both species, which have been collected during the period 2000-2008 by National Environmental Reseach Institute (NERI). These data was supplemented by acoustic data on Harbour porpoises recorded inside ad outside the project area and measurements of background subsea noise performed in the period 16 June to 16 August. In addition, data from aerial counts (total numbers) of seals on the haul-out sites Bosserne, Møllegrund, Hesselø and Læsø and Encounter rates (n/km) of Harbour porpoise in the area between Djursland, Læsø and Anholt obtained by 16 aerial waterbird line transect surveys between 1999 and 2006 were used to the assessment.
The major gradients in the suitability of habitats for Harbour porpoises and Harbour seals in various parts of the region were estimated on the basis of the aerial survey data and the telemetry data. As the two data sets represent data with strikingly dif ferent characteristics (telemetry=presence data, aerial surveys=presence/absence data) a robust statistical method was applied to model the mean habitat suitability of the region. To correlate the environmental variables of the area to the presence data of marine mammals a spatial modelling technique called Ecological Niche Factor Analysis (ENFA) was applied.
While differences between measurements with similar conditions were small, a big difference in background noise could be observed for varying maritime traffic. In other words, it can be expected that the ambient noise is influenced by ship traffic, especially the ferry traffic.
The observations of Harbour porpoises from the aerial surveys were biased towards the deeper and more pelagic south-easterly part of the region, a situation which clearly affected the modelled habitat suitability for Harbour porpoise. The modelled habitat suitability on the telemetry data indicates that the southern-central part of the region and the area north of Anholt is used more intensively than the shallower areas with lower salinity and more flat terrain. Although single records of porpoises were located in the latter type of areas, the satellite fixes generally fall within the predicted areas of high suitability. Accordingly, the habitat suitability of the Project Area classifies as medium to high within the range of habitat quality to porpoises found in the north-western Kattegat.
The click train indices (hourly DPM) show that Harbour porpoises were present in the Project Area throughout the summer period. Maximum and mean DPM-values were generally largest for stations 2 and 4, and smallest for stations 5 and 6. Maximum DPM levels for stations 2 and 4 were close to 50 %. Mean DPM values for stations 2 and 4 were 1-2 per hour, while they were less than 1 DPM per hour for the other stations, Table 3-3. The DPM values indicate an abundance of porpoises which may be considered as intermediate between the coastal North Sea (higher abundance) and the western Baltic (lower abundance).
The modelled habitat suitability of all records of Harbour seal satellite telemetry activities in the north-western Kattegat resulted in relatively clear estimates of the trends in habitat use of the species. The modelled habitat suitability values indicate clearly that a coherent area of high suitability is aligned north-south off the Totten colony and a smaller but well-defined is located just east of the Project Area. The Project Area itself seems to be unsuitable for Harbour seals coming from the Totten colony.
The impacts due to subsea noise emissions during the construction phase are assessed as moderate for all three species of marine mammals. Taking all possible uncertainties into account the assessment of impacts due to underwater noise emis- sion during construction concluded that a zone of audibility will extend between 20 and 80 km from the source for the species. At the Project Area, background noise is 100 dB rms at 2 kHz (1/3 octave band). Frequencies higher than app. 2 kHz will be below background noise and porpoises and seals will most likely not detect them at large distances (> 50 km). A wide zone of responsiveness in Harbour porpoises and Harbour seals is estimated. As a realistic estimate, the responsive radius can be de- fined as at least 20 km from the construction site. For the entire Project Area of the Anholt OWF the range of 20 km will cover areas of intermediate habitat suitability to Harbour porpoises and high habitat suitability to Harbour seals in the Kattegat. How- ever, these effects should be of short duration, allowing the animals to return to the areas of origin following pile driving activities. Masking of communication may occur in Harbour porpoises and seals over distances of more than 20 km from the source, yet the effect is assessed to be small. Temporal hearing loss (TTS) might occur at 1,000 m in Harbour porpoises and 250 m in seals.
Other impacts during construction are considered as minor. Noise from ships associated with the construction activity could lead to responsive reactions in Harbour porpoises and at close range (2-300 m).
During operation only minor impacts are envisaged. The results indicate a rather small zone of audibility and noise levels, at ranges smaller than 1,000 m are too low to induce responsiveness, masking or TTS in porpoises. There might be masking of Harbour seal sounds but this will happen at close ranges well below 1 km.
The potential major impacts related to the potential TTS zone during pile-driving operations can be mitigated, while the overall moderate impacts due to short-term responsive movements may be impossible to mitigate. A range of mitigation measures are recommended.
Regarding operational noise from the planned Universal Wind OWF and suspension of sediments, traffic and electromagnetic fields, no cumulative effects on marine mam- mals is expected.